All it took was a relationship breakdown and a shoulder injury for Ian Gardiner's circumstances to descend from a comfortable life on Sydney's northern beaches to sleeping in the back of his Nissan Pulsar in a backpacker hostel carpark.

The single father of three sons had been earning a reasonable income as a self-employed car sales consultant when an injury meant he could no longer work.

When the owner of his Dee Why apartment decided to sell at the end of 2012, the 56-year-old found himself in dire circumstances.

"I didn't have enough money for a bond because I hadn't been working plus there was limited availability over that time," he said.

"I ended up moving into a backpacker's hostel in Collaroy. I didn't have any other option. I hated it. I ended up moving into the back of my car, a Nissan Pulsar hatchback, that's what I lived in."

Six of Australia's largest welfare groups have written to the country's major political parties, drawing attention to the growing number of people like Mr Gardiner who simply fall into homelessness through sheer bad luck and lack of affordable housing.

Mission Australia, Anglicare, UnitingCare, the Salvation Army, Wesley Mission and St Vincent de Paul have joined forces to propose ways in which Australian's 105,000 homeless figure can be halved over the next decade.

Their call has been echoed by peak body Homelessness Australia which launched an online petition on Tuesday.

After almost a year of living in his car, Mr Gardiner became gravely ill with a perforated bowel and peritonitis. He underwent surgery which had severe complications, resulting in him spending almost two months in hospital and losing 28kg from his 90kg frame. He applied for social housing but being classed as a single male put him low on the priority list.

"It was scary," he said. "There was nowhere for me to go."

Following another round of surgery, Mr Gardiner moved into a men's shelter run by Mission Australia near Manly. He recuperated and eventually moved into social housing in Willoughby last year with his only possessions - a fan, bar fridge, bag of clothes and his car.

"It's amazing having a place to call my own," he said. "My health is improving, it's safe and secure. Now I just want to get back to work."

The joint letter from the welfare groups points out that almost 15 per cent of Australia's homeless are over 55 and 40 per cent are under 25.

"In a prosperous nation like Australia there are adequate resources to ensure everyone has a home," Mission Australia chief executive Catherine Yeomans said. "Yet we have unacceptably high, and growing, rates of homelessness."

A Federal budget decision to only extend a national homelessness agreement until mid-2017 has prompted concern, with the welfare sector inviting political leaders to attend a forum ahead of the election to hear proposals for reducing the number of homeless.

Mr Gardiner said his story illustrates just how easy it is for anyone to be tipped into homelessness.

"It just went downhill very, very rapidly," he said. "I ended up losing everything."